1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image using toner, and a technique for calculating a toner consumption amount in the image forming apparatus.
2. Description of the Related Art
In an image forming apparatus, such as a printer, a copier machine and a facsimile machine, which forms an image using toner, it is necessary to grasp a consumption amount or the remaining amount of toner, for maintenance purposes such as to supply toner. Noting this, in Japanese Patent Application Laid-Open Gazette No. 2002-174929, a method of and an apparatus for detecting a toner consumption amount has been disclosed which permit, by means of a simple structure, to accurately calculate the amount of toner which is consumed as a toner image is formed in a predetermined unit (e.g., in the unit of a page, a job, etc.).
Considering that a relationship between the values of print dots and a toner consumption amount is non-linear and that the non-linear relationship changes also in accordance with the states of dots which are adjacent to this print dots, this detection method and the detecting apparatus demand to classify a string of print dots into three patterns of isolated dots, consecutive double dots and intermediate value dots, count the number of dots forming each pattern and calculate a toner consumption amount based on thus obtained counts.
By the way, although the method and the apparatus described in Japanese Patent Application Laid-Open Gazette No. 2002-174929 allow to calculate a toner consumption amount during an ordinary image forming operation based on print dots, the method and the apparatus give no consideration on an operation under a non-ordinary mode which is different from the ordinary image forming operation. However, an operation which will eventually lead to a consumption of toner could be triggered even during execution of the non-ordinary mode operation. Hence, there is a first problem that it is not possible to accurately calculate a toner consumption amount when no consideration is given on such an operation.
Further, the only route illustrated in FIGS. 2 and 4 of Japanese Patent Application Laid-Open Gazette No. 2002-174929 mentioned above as a route for inputting a signal to a laser driver is a route for inputting pulse signals obtained by modulating print dots by a pulse modulating circuit. Despite this, an image forming apparatus may have such a structure that there are multiple of routes for feeding signals to a laser driver which serves as image forming means. An example is an image forming apparatus having a structure in which there is another route for inputting a signal which is irrelevant to print dots in addition to the above-mentioned route which is relevant to print dots (hereinafter referred to as “the print-dot route”), to thereby form an image which is different from the print dots.
When such an image forming apparatus receives a signal through the print-dot route mentioned above and performs an image forming operation based on print dots, the amount of toner which is consumed in the image forming operation can be calculated according to the method and as in the apparatus described in above-mentioned Japanese Patent Application Laid-Open Gazette No. 2002-174929. However, when an image forming operation which is not based on print dots is executed after reception of a signal through another route mentioned earlier, the method and the apparatus described in Japanese Patent Application Laid-Open Gazette No. 2002-174929 do not allow to calculate the amount of toner which is consumed in the image forming operation. In consequence, there is a second problem that it is impossible to accurately calculate a toner consumption amount in the image forming apparatus as a whole.
In addition, as described above, the method and the apparatus described in Japanese Patent Application Laid-Open Gazette No. 2002-174929 demand to classify a string of print dots into three patterns of isolated dots, consecutive double dots and intermediate value dots, count the number of dots forming each pattern, calculate the consumption amounts of toner in the respective colors recorded on a recording paper based on thus obtained counts, add an offset amount to these, and accordingly calculate the total amount of toner of the respective colors consumed at this stage. As for the offset amount, Japanese Patent Application Laid-Open Gazette No. 2002-174929 describes that “an offset amount is the amount of toner which is consumed independently of an exposure time with laser light, and as such, a unique value to each color image forming apparatus.” In other words, the offset amount mentioned above is a constant value. Therefore, the offset amount which is a constant value is added to the toner consumption amounts calculated based on the counts described earlier, whereby the total amount of the consumed toner are calculated.
By the way, in recent image forming apparatuses, in an attempt to improve the convenience of use, an engine section (image forming means) which performs formation of an image is provided with an operation signal containing various information from a host computer or a controller such as a main controller which deciphers a print command signal fed from the host computer. This gives rise to a third problem that in such an image forming apparatus, when an operation sequence, an operating state or the like of the engine section changes in response to the operation signal, if the offset amount is fixed to a constant value as in the case of the method and the apparatus described in Japanese Patent Application Laid-Open Gazette No. 2002-174929, it may not be possible to accurately calculate the amount of consumed toner.
Further, as described above, the method and the apparatus described in Japanese Patent Application Laid-Open Gazette No. 2002-174929 demand to classify a string of print dots into three patterns of isolated dots, consecutive double dots and intermediate value dots, count the number of dots forming each pattern and calculate the total amount of toner which constitutes a toner image (hereinafter referred to as “image constituting toner”) based on thus obtained counts.
Still further, considering that there is toner which gets consumed separately from image constituting toner during formation of a toner image, an offset value (unique value) is added to the total amount mentioned above and the resultant value is used as a toner consumption amount. That is, as is already known in the art, even during execution of an image forming operation to form a white image, i.e., to form no print dot at all, so-called fogging occurs and a small amount of toner is consumed. Noting this, the amount of thus consumed toner is added, to thereby improve the accuracy of calculating a toner consumption amount.
In the case of such an image forming apparatus, to stably form a toner image, it is desirable that characteristics of toner to use remain constant. However, it is known that in an actual apparatus, as toner images are formed repeatedly, the image density of a toner image could sometimes gradually change. Characteristics of toner are thus not always constant but could change with time. How this change occurs is different depending on the structure of an apparatus or toner to be used. For instance, this type of image forming apparatus accompanies a phenomenon called “selective development,” i.e., a phenomenon that in the case of toner containing particles having various particle diameters, toner having certain particle diameters is selectively consumed during development. Due to this, a particle diameter distribution of remaining toner gradually changes. Changes of toner characteristics with time of course influence the quality of a toner image which is formed, and also brings about changes of an offset value mentioned earlier.
It is also known that in this type of image forming apparatus, the quality of an image such as the density of the image is controlled, as image forming conditions are changed which consist of various factors such as a bias potential which is applied upon each portion of the apparatus. In addition, the image density of a toner image may change owing to a difference between individual apparatuses, a change with time, a change in environment surrounding the apparatus such as a temperature and a humidity level, etc. Therefore, image forming conditions which are influential over image densities among those factors are adjusted, thereby controlling image densities. The amount of fogging also changes as image forming conditions are changed, and an offset value also changes as the image forming conditions are changed.
Once the offset value has changed, in the case of a conventional image forming apparatus in which the offset value is to be fixed, a calculated toner consumption amount becomes different from an actual amount and it could therefore become difficult to supply toner at proper timing. Here arises a fourth problem to provide a technique which permits to calculate a toner consumption amount at a higher accuracy regardless of a change with time of the offset value.
By the way, over the recent years, capabilities of color image forming apparatuses have improved and there now is a risk that unauthorized use could be made of these improved apparatuses. A technique which has been proposed in an effort to prevent unauthorized printing against this background is to add, to an image to be printed with an image forming apparatus, namely, an original image, a special image which identifies this image forming apparatus or specifies a person who has printed. As shown in FIG. 26 for instance, in the event that one wishes to print in colors a map containing a confidential item on a sheet S such as a transfer paper, a copy paper and a sheet for overhead projector (hereinafter referred to as “OHP sheet”), among output color components (which are magenta, cyan, yellow and black for example) available in the image forming apparatus, one which is least noticeable to human eyes (yellow, for instance) may be used to print a special image S1 which expresses a serial production number of the image forming apparatus or the like.
In the case of an image forming apparatus capable of printing a special image S1, a special image S1 is printed over an original image in some instances. As compared to where an original image alone is printed, toner of the output color component which is least noticeable to human eyes is consumed in the amount equivalent to the printing of the special image S1. Hence, there is a fifth problem that a direct application of the toner consumption amount calculation technique implemented in such a conventional apparatus which is supposed to print an original image alone would not make it possible to accurately calculate the consumption amount of toner which constitutes a special image S1.